Bionomial theorem/ question (wp)

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Homework Help Overview

The discussion revolves around finding the values of k for the function f(x) = x^3 + 6x^2 + kx - 4, such that it gives the same remainder when divided by x - 1 and x + 2. Participants are questioning the correctness of their calculations and the reasoning behind the expected answer of 3 as stated in a booklet.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants attempt to set up equations using the Remainder Theorem by evaluating f(1) and f(-2). There is confusion regarding the steps taken to equate the remainders and the implications of their calculations. Some participants express uncertainty about the disappearance of terms and the need for substitution in their equations.

Discussion Status

There is ongoing exploration of the problem with various participants attempting to clarify their understanding of the equations involved. Some guidance has been offered regarding the need to set the two remainders equal and substitute appropriately, but there is still a lack of consensus on the correct approach and calculations.

Contextual Notes

Participants are working under the constraints of homework rules, which may limit the information they can share or the methods they can use. There is also a noted discrepancy between their findings and the answer provided in the booklet, leading to further questioning of their assumptions and calculations.

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Homework Statement




For what values of k does the function f(x)= x^3 + 6x^2 +kx -4 give the same remainder when divided by x-1 and x+2?

answer, however in the booklet it says the answer is 3, why is this? what did i do wrong?

Homework Equations





The Attempt at a Solution



So.. i set up two equations one using f(1) and one using f(-2)

f(1)= 1 + 6 + 1k -4
f(1) = -2 +k
2 = k

f(-2) = 12 -2k
2k = 12
k= 6,

I plugged 6 and 2 into the equation of f(1) and got the same (remainder of 9)

wat am i doing wrong?
 
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anyone??
 
Nelo said:
anyone??
Patience !

Please, wait at least 24 hours before bumping your thread.
 
Nelo said:

Homework Statement

For what values of k does the function f(x)= x^3 + 6x^2 +kx -4 give the same remainder when divided by x-1 and x+2?

answer, however in the booklet it says the answer is 3, why is this? what did i do wrong?

Homework Equations


The Attempt at a Solution



So.. i set up two equations one using f(1) and one using f(-2)

f(1)= 1 + 6 + 1k -4
f(1) = -2 +k This says that the remainder for f(x)/(x-1) is k - 2 .
2 = k Why do this step?

f(-2) = 12 -2k This says that the remainder for f(x)/(x+2) is 12 -2k .
2k = 12
k= 6,

I plugged 6 and 2 into the equation of f(1) and got the same (remainder of 9)

what am i doing wrong?

What does the remainder theorem say?

Set the two remainders equal to each other. Solve for k.
 
wat? like.. 6k = 2?

6/2 = 3
k=3?
 
Nelo said:
So.. i set up two equations one using f(1) and one using f(-2)

f(1)= 1 + 6 + 1k -4
f(1) = -2 +k
1 + 6 - 4 = -2?

2 = k

f(-2) = 12 -2k
2k = 12
k= 6,
Where did the f(1) and f(-2) go? This makes no sense. If you're supposed to get the same remainder when dividing by x - 1 and x + 2, then that means that
f(1) = f(-2).

And can you also please double check your work before submitting your post?
 
f(1)= 1 + 6 + 1k -4
f(1) = 7-4+1k
f(1) = 3+k
-3=k

f(-2) = (-2)^3 + 6(-2)^2 + k(-2) -4
f(-2) = 12 -2k
2k = 12
k= 6

?? what do i do now
 
Nelo said:
f(1)= 1 + 6 + 1k -4
f(1) = 7-4+1k
f(1) = 3+k
-3=k
Again, why are you making the f(1) disappear? DON'T DO THAT! :mad:

You say
f(1) = 3 + k
and
f(-2) = 12 - 2k.

Since the remainders are supposed to be equal,
f(1) = f(-2)
so substitute with the right-hand-side of the two previous equations.
 
I don't get it... substitute something that's not a full equation into the right hand side of the prev 2 eq? doesn't something have to = k inorder for me to sub it into another equation..?

How do i substitute dat
 
  • #10
Nelo said:
I don't get it... substitute something that's not a full equation into the right hand side of the prev 2 eq? doesn't something have to = k inorder for me to sub it into another equation..?

How do i substitute dat
f(1) = f(-2)
Instead of f(1), write what it equaled to.
Instead of f(-2), write what it equaled to.
That's what I meant by substituting.
 
  • #11
f(1) = 3 + k
and
f(-2) = 12 - 2k.


3+k = 12-2k
-9 = -3k
k= 3 ...

I see, thanks :P
 
  • #12
That's exactly what I suggested you do back in post #4 .
 
  • #13
gl bk
 

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